Due to recent mechanical advances and keen attention paid to maintenance-free benefits, lubrication by means of grease is required to be performed under more and more severe conditions. Thus, use of a grease composition which is stable particularly under high-temperature conditions for a long period of time is a key to prolong of service life of machines. Therefore, there is demand for enhancing performance of such a grease composition.
In electronic parts and devices which have been remarkably developed in recent years as well as in production facilities therefor, contamination of grease decomposition products is problematic. Therefore, there is also demand for low-evaporative grease compositions.
Grease compositions exhibiting a long service life under high-temperature conditions which have heretofore been proposed include a grease comprising a trimellitate ester as a base oil (see, for example, Patent Documents 1 to 3) and a grease comprising alkylated diphenyl ether as a base oil (see, for example, Patent Document 4). In addition, greases comprising methylsilicone or methylphenylsilicone as a base oil, greases comprising perfluoroalkyl polyether (hereinafter referred to as “PFAE”) as a base oil, and other grease products have also been proposed. Among them, grease products comprising PFAE as a base oil, which exhibit a long service life under high-temperature conditions, are generally employed in an atmosphere of a temperature higher than 200° C.
However, when grease products comprising PFAE as a base oil are employed under higher-temperature conditions, fluoro-oil contained in the products is decomposed, which limits use of the grease products under such high-temperature conditions. Therefore, at present, grease replenishment intervals or exchange cycles of lubricated parts are shortened.
Among grease compositions exhibiting a long service life under the aforementioned high-temperature conditions, a grease product comprising PFAE as a base oil has resistance to decomposition up to a certain high temperature. Thus, such a PFAE-comprising grease product is effectively employed as a low-evaporative grease. However, performance of such a grease is still unsatisfactory, and there is also demand for a grease composition which is low-evaporative under higher-temperature conditions.
Although most grease compositions are classified as a non-hazardous substance by the fire services law, a grease composition comprising a base oil originating predominantly from carbon, hydrogen, and oxygen is combusted when used in environments which allow direct contact with fire or used along with high-temperature metallic material. When a grease comprising PFAE—incombustible base oil—is used under such conditions, toxic gas may be generated through decomposition of the grease at high temperature. A grease comprising silicone oil as a base oil does not cause such a problem, but provides reduced lubricity under boundary lubrication conditions, which is problematic.
[Patent Document 1]
    Japanese Patent Publication (kokoku) No. 7-45677[Patent Document 2]    Japanese Patent Application Laid-Open (kokai) No. 11-131082[Patent Document 3]    Japanese Patent Application Laid-Open (kokai) No. 7-109480[Patent Document 4]    Japanese Patent Application Laid-Open (kokai) No. 3-28299